A liquid crystal display device is widely utilized for a display of a personal computer, a television and so forth. The display device utilizes dielectric anisotropy, optical anisotropy or the like of a liquid crystal compound. As an operating mode of the display device, various modes are known, such as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode.
In such a liquid crystal display device, a liquid crystal composition having suitable physical properties is used. In order to further improve characteristics of the display device, the liquid crystal compound contained in the composition preferably has physical properties as shown in (1) to (8) below:
(1) high stability to heat, light and so forth;
(2) high clearing point;
(3) low minimum temperature of a liquid crystal phase;
(4) small viscosity (η);
(5) large value of dielectric anisotropy (Δ∈);
(6) suitable value of optical anisotropy (Δn);
(7) suitable elastic constant (K); and
(8) excellent compatibility with other liquid crystal compounds.
An effect of the physical properties of the liquid crystal compound on the characteristics of the liquid crystal display device is as described below.
A compound having a high stability to heat, light and so forth as described in (1) increases a voltage holding ratio of a device. Thus, a service life of the display device becomes long. A compound having a high clearing point as described in (2) extends a temperature range in which the device can be used. A compound having a low minimum temperature of a liquid crystal phase such as a nematic phase or a smectic phase as described in (3), particularly, a compound having a low minimum temperature of the nematic phase also extends the temperature range in which the device can be used. A compound having a small viscosity as described in (4) shortens a response time of the device.
A compound showing a large dielectric anisotropy as described in (5) decreases a threshold voltage of the display device. Thus, an electric power consumption of the display device becomes small. As is well known, the threshold voltage (Vth) is expressed by an equation as described below (H. J. Deuling, et al., Mol. Cryst. Liq. Cryst., 27 (1975), 81).Vth=π(K/∈0Δ∈)1/2 
In the equation, K is an elastic constant of a liquid crystal material, and ∈0 is a dielectric constant of vacuum.
A compound showing a suitable optical anisotropy as described in (6) improves a contrast of the display device. According to a design of the display device, a compound showing a large optical anisotropy or small optical anisotropy, more specifically, a compound showing a suitable optical anisotropy is required. When a response time is shortened by decreasing a cell gap of the display device, a compound showing a large optical anisotropy is suitable.
With regard to (7), a compound having a large elastic constant shortens a response time of the display device, and a compound having a small elastic constant decreases a threshold voltage of the display device. Accordingly, a suitable elastic constant is required according to characteristics to be desirably improved.
A compound having excellent compatibility with other liquid crystal compounds as described in (8) is preferred. The reason is that such a compound is useful in view of adjusting physical properties of a liquid crystal composition by mixing two or more kinds of liquid crystal compounds having different physical properties.
Various kinds of liquid crystal compounds showing a large dielectric anisotropy and a large optical anisotropy have been synthesized so far, and some of the compounds have been put in practical use.
For example, Patent literature Nos. 1 to 3 disclose a compound having a CF2O bonding group. However, the compounds have an insufficiently high value of dielectric anisotropy in view of requirements of a marketplace. Therefore, when a liquid crystal composition containing each of the compounds is prepared, Vth cannot be sufficiently decreased in view of the requirements of the marketplace.
Furthermore, Patent literature Nos. 4 to 5 discloses a tricyclic compound (compounds (S-1) to (S-2)) having two CF2O bonding groups. However, compound (S-1) has a low phase transition temperature between a liquid crystal phase and an isotropic phase, and compound (S-2) has a small value of optical anisotropy.
Moreover, Patent literature Nos. 6 to 7 disclose a pentacyclic compound having two CF2O bonding groups (compounds (S-3) to (S-4)). However, the compounds (S-3) to (S-4) have an insufficiently small viscosity in view of requirements of a marketplace.
